As saving energy becomes more important, it has become desirable to reduce the energy consumption associated with widely used lighting systems and light fixtures, which fixtures can include HID luminaires. At present roadways, highways and residential streets are fully lit throughout the night, despite lighter traffic use between midnight and dawn.
HID lamps, and their lamp fixtures or HID luminaires, are typically used when a high level of light over a large area is required, and when energy efficiency and/or long life are desired. Uses that are well suited for such HID luminaries include gymnasiums, large public areas, warehouses, buildings, signs, outdoor activity areas, sports fields, roadways, parking lots, and pathways. More recently, however, HID sources, especially metal halide, have been used in small retail and residential environments. Conservative estimates suggest there are at least 150 million HID luminaires worldwide on roadways alone.
HID lamps—which includes mercury vapor (MV) lamps, metal halide (MH) lamps, high-pressure sodium (HPS) lamps, low-pressure sodium lamps, and less common, xenon short-arc lamps-have light-producing elements that use a well-stabilized arc discharge contained within a refractory envelope (arc tube).
Light fixtures can have their lumen output adjusted to save energy when full brightness is not needed, such as when lighted areas are not occupied, or during periods of reduced usage. Full brightness can be restored when increased occupancy is detected.
However, there are several problems and difficulties with adjusting and controlling the lumen output of HID lamps. One of the reasons for the problems is that ballasts are required to start the lamp, to regulate the lamp starting and lamp operating currents, and to sustain an appropriate supply voltage. A first problem is that HID lamps require several minutes to ignite, warm-up, and reach their full light output levels. Second, HID lamps also have a hot re-strike problem, which makes them difficult to re-ignite within a short period after being turned off, while they are still at an elevated temperature. Depending upon the ballast-lamp combination, it can take up to 10 minutes after the lamp has been turned off before it can be re-ignited. This poses a practical problem for lamp adjustment applications involving pedestrian conflict or roadway safety where the lamp must be returned, within a very short period of time (i.e. within milliseconds), to an acceptable level of brightness and illumination. If the HID lamp is turned off, it may take several minutes after re-ignition until the lamp warms up sufficiently to produce the desired output.
Several methods can be used for adjusting the lumen output, and the power consumption, of ballasted HID luminaires. A first method uses a variable voltage transformer to reduce the primary voltage supplied to the ballast, thereby achieving lumen adjustment to approximately 60% of the rated lamp power. Typically, in this type of arrangement, an HID luminaire contains a lamp as well as some type of transformer ballast with a series-connected inductance and capacitance (L-C circuit), in the form of a choke and capacitor, for controlling the lamp operating current according to the voltage-current characteristics specified for the ballast-lamp combination.
A second method uses a variable reactor in the ballast circuit to change the lamp current without affecting the voltage. This method allows a wider range of lumen and power adjustment, permitting a reduction to approximately 30% of rated power, depending upon the lamp and ballast combination.
A third method for adjusting lumen output uses solid-state components to change the waveforms of both the current and voltage input to the ballast, which permits adjusting lumen output down to approximately 50% of rated power.